1. Field of the Invention
The present invention relates to a method of irradiating an object as a target of irradiation (hereinafter simply referred to as the target object) through the use of a laser beam (hereinafter, referred to as laser annealing) and to a laser beam irradiating apparatus (an apparatus including a laser and an optical system for guiding a laser beam outputted from the laser to the target object) for performing the laser beam irradiating method. The invention also relates to a method of manufacturing a semiconductor device manufactured through the fabrication sequence including the laser beam irradiating method. The semiconductor device referred to herein means general devices designed to function by exploiting the semiconductor characteristics, and includes, for example, electro-optic devices, such as a liquid crystal display and a light emitting device, and electronic equipment incorporating the electro-optic devices as a component.
2. Description of the Related Art
Recently, an extensive study has been conducted for a technique of applying laser annealing to a semiconductor film formed on a insulating substrate, for example, made of glass, to crystallize the semiconductor film or to improve the crystalline characteristics thereof. Silicon is often used for the semiconductor film. In the present specification, means to obtain a crystalline semiconductor film by crystallizing the semiconductor film through the use of a laser beam is referred to as laser crystallization.
A glass substrate is advantageous over a conventionally popular synthetic quart glass substrate in that it is inexpensive and easy to process, which makes it easier to manufacture a large-scale substrate. This is the reason why the aforementioned study has been conducted. Meanwhile, the laser is preferably used for crystallization because of a low melting point of the glass substrate. That is, the laser can confer high energy to the semiconductor film alone without substantially raising the temperature of the substrate. Also, the laser can attain markedly high throughput in comparison with heating means using an electric heating furnace.
A crystalline semiconductor film formed through irradiation of a laser beam has a high mobility. For this reason, thin film transistors (TFTs) are formed from this crystalline semiconductor film, and these TFTs are, for example, incorporated in an active matrix liquid crystal display as TFTs for the pixel portions, or for both the pixel portions and the driving circuit portions provided on a single glass substrate.
A laser beam emitted from an Ar laser, an excimer laser, etc. is often used as the aforementioned laser beam. A method of performing laser crystallization through the use of an Ar laser is disclosed in, for example, Japanese Patent Laid-Open Nos. 163401/1994 and 326769/1995.
The excimer laser has an advantage that it has a large output and is able to repetitively irradiate a laser beam at a high frequency. Laser beams emitted from the foregoing lasers have an advantage that they have high absorption coefficients with respect to a silicon film used often as the semiconductor film.
In regard to laser beam irradiation, a highly productive and industrially excellent method can be achieved by configuring an optical system in such a manner that a laser beam forms an elliptic shape, a rectangular shape, or a linear shape on the irradiation surface or in the vicinity thereof, and irradiating such a laser beam while moving the laser beam (alternatively, moving the irradiating position of the laser beam relatively with respect to the irradiation surface). It should be noted that the term “linear shape” referred to herein does not mean an exact, sharp line; it means a rectangle (or oblong) having a large aspect ratio. For example, it means those having an aspect ratio of 10 or greater (preferably, 100 to 10000). In the present specification, a laser beam forming an elliptic shape on the irradiation surface is referred to as an elliptic beam. Likewise, a laser beam forming a rectangular shape is referred to as a rectangular beam and the one forming a linear shape as a linear beam.
Generally, in regard to a laser beam forming an elliptic shape, a rectangular shape, or a linear shape on the irradiation surface or in the vicinity thereof through the optical system using no beam homogenizer, the energy density reaches the peak at the center and attenuates gradually in the edge portions due to aberration of the lens (FIG. 8). With such a laser beam, a region having the sufficient energy density to irradiate a target object accounts for an extremely small proportion of the laser beam, approximately ⅕ to ⅓ including the center thereof. The regions in the edge portions of the laser beam having insufficient energy density to irradiate a target object are defined herein as attenuated regions.
A longer elliptic beam, linear beam or rectangular beam is formed to meet an increase in the area of the substrate or in the output of the laser. This is because irradiating a longer laser beam can attain better efficiency. However, the energy density in the edge portions of a laser beam emitted from the laser is small in comparison with that in the vicinity of the center, and there is a tendency that expanding a laser beam further through an optical system makes the attenuated region more noticeable.
The attenuated region has insufficient energy density in comparison with that at the center of a laser beam, and therefore, when a laser beam having the attenuated regions is irradiated, a target object cannot be irradiated in a satisfactory manner.
To be more specific, when the target object is a semiconductor film, the resulting crystalline semiconductor film has different crystalline characteristics between a portion irradiated by the attenuated regions and a portion irradiated by the other region, including the center, having high energy density. Hence, when TFTs are manufactured from such a semiconductor film, the TFTs manufactured from the portion irradiated by the attenuated regions have poor electrical characteristics, which causes unwanted variance in a single substrate.